Aaron of the Old Testament held many important roles among the Elders of his day. The Lord first gave a lesser Priesthood in order to perform certain ordinances. The Aaronic Priesthood is sometimes called the Levetical Priesthood. The Aaronic Priesthood is passed on from worthy Father to son, unlike the Melchizedek Priesthood, which is conferred by the laying on of hands from a worthy Mormon male.
John the Baptist had the Aaronic Priesthood when he baptized Jesus because he was a direct descendant of Aaron. The Mormon church claims that John the Baptist appeared to Joseph Smith and Oliver Cowdery and conferred to them the Aaronic Priesthood, also known as the lesser priesthood.
The Aaronic Priesthood has the power to call upon and receive visitation from angels. This literally means any Mormon male who has the Aaronic Priesthood has the power to call upon heavenly messengers and angels. In the church today, this power is not commonly practiced or discussed among LDS members. Other principles, such as faith, repentance, and baptism also fall under the powers of the Aaronic Priesthood. However, in order to receive the Holy Ghost, one must have the Melchizedek Priesthood.
If a worthy male lives righteously and remains true and faithful they will be interviewed to receive the greater priesthod. The greater priesthood is called the Melchizedek Priesthood, which gives them the power to officiate greater Mormon ordinances.